Method for Cooling a Detector

ABSTRACT

A detector, in particular an IR detector in the seeker head of a guided missile, is cooled with expanding gas generated by depressurizing a pressurized fluid. A mixture which forms a positive azeotrope, including argon or nitrogen as a main component and at least one alkane as a secondary component, is expanded as the fluid. The composition is preferably in the region of a eutectic mixture in order to avoid a component freezing adjacent to the expansion nozzle. This makes it possible to considerably extend the life of the cooler in comparison to that when using pure cooling gases such as nitrogen or argon, and to cool the detector down more quickly, subject to the same constraints.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority, under 35 U.S.C. §119, of Germanpatent applications DE 10 2007 004 999.6, filed Feb. 1, 2007 and DE 202007 008 674.1, filed Jun. 21, 2007; the prior applications are herewithincorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for cooling a detector, in particularan IR detector in the seeker head of a guided missile. The expanding gasafter a pressurized fluid has been expanded is used to cool thedetector.

A method such as this uses the so-called Joule-Thomson effect (alsoreferred to as the Joule-Kelvin effect) for cooling, making use of theexpansion of a real gas, which is not the same as that of an ideal gas.When a real gas is expanded below its inversion temperature, then it iscooled down because of the positive Joule-Thomson coefficient—that is tosay by means of certain interactions between the gas molecules. This gaswhich has been cooled down and expanded in turn further cools down theinlet high-pressure gas, by means of a counter-flow heat exchanger,until the expanding high-pressure gas exists in the vapor area of thereverse-flow heat exchanger below the boiling or dewline—that is to sayin the wet vapor region—partially as a liquid, condensed phase andpartially as a gaseous phase. This cooling down of the gas to theboiling point of the respective real gas with expansion to about 1 barbelow its inversion temperature is made technical use of in a versatilemanner in order to produce low temperatures or to cool downtemperature-sensitive equipment, in particular such as detectors. Inparticular, optical detectors in the infrared range, so-called IRdetectors, must be cooled down to temperatures of below 100 K in orderto achieve a good signal-to-noise ratio.

In the case of expansion or Joule-Thomson coolers based on theJoule-Thomson effect, a suitable pressurization working gas(high-pressure cooling gas) is expanded by means of a restrictor ornozzle, and the emerging gas which has been cooled down by virtue ofisoenthalpy expansion is used to reduce the temperature of the gas inletand of a detector that is disposed adjacent to the expansion nozzle. Inthis case, the expanding and partially liquefying gas is aimed directlyagainst the detector rear wall to be cooled, or against a thermallyhighly conductive intermediate wall to which the emerging gas can beapplied and on whose front face the detector is arranged. In order toreach the boiling point of the cooling gas and to achieve good coolingperformance at this respective boiling point of the respective gas, itis known for the working gas which is supplied on the high-pressure sideto the expansion nozzle—also referred to as a restrictor—to be cooleddown by flowing in the opposite direction to the inlet working gas,which is being expanded, through an appropriate reverse-flow heatexchanger before finally emerging into the surrounding area.

Particularly when the installation conditions are confined and theenergy resources are low, it is impossible to operate a Joule-Thomsoncooler in a closed cooling circuit, for example on the basis of theLinde process, with the expanded working gas being compressed with heatbeing emitted, and being supplied in a pressurized form to the expansionnozzle once again, in order to cool down the detector. This is becausecompressors require physical space and a large amount of energy foroperation, whose dissipated heat must also be dissipated. So-called openJoule-Thomson coolers are known for confined operational conditions suchas these, in which the pressurized working gas is emitted to thesurrounding area from a high-pressure gas container after it has beenexpanded, after the required detector cooling and after flowing back inthe reverse-flow heat exchanger. Open Joule-Thomson coolers such asthese are used, for example, to cool IR detectors in the seeker heads ofguided missiles where neither physical space nor energy are available toallow a compressor system to be used for a closed Joule-Thomson cooler.A two-stage, open Joule-Thomson cooler for cooling an IR detector in theseeker head of a guided missile is described, for example, in Europeanpatent specification EP 0 432 583 B1 and U.S. Pat. No. 5,150,579.

Since, for obvious reasons, the amount of working gas which can be madeavailable from a high-pressure vessel in a missile is restricted, andsome of the carrier aircraft cannot supply high-pressure working gasfrom the aircraft or launcher, also referred to as the launch row, onlya limited supply of working gas can be carried in a pressure bottle,either for rapid cooling of an IR detector in the missile itself, forlong-lasting detector cooling, for greater cooling power levels or forcombinations thereof.

The volumes of high-pressure bottles with a maximum volume of up to 500cm³ carried in conventional air-to-air missiles and which are restrictedfor physical space reasons are disadvantageously suitable only for arestricted cooler running time for cooling an IR detector. In the statedconditions and depending on the ambient temperature, cooler runningtimes of merely between 1.5 and 3 hours can currently be achieved usingair, nitrogen or argon as the working gas, which make it possible toreduce the temperature of the IR cooler to below the 100 K boilingpoint. New types of operational scenarios for modern combat jets arenow, however, based on flying times which may be from 6 to 8 hours,during which the IR detector in the missile must be kept at a cryogenicoperating temperature of below 100 K, for the missile to be ready foroperation at all times.

In other applications with short cooler running times, detectorcooling-down times of less than 1.5 to 2 seconds must be achieved bymeans of the Joule-Thomson cooler from the restricted high-pressurebottle volumes of a few cubic centimetres, but at high gas pressures.This is the situation, for example, with portable surface-to-airmissiles against enemy combat aircraft (so-called “Manpads”—ManportableAir Defense Systems—and with marine missile defense systems (againstso-called Seaskimmers—missiles approaching at low level) and enemycombat aircraft. In certain circumstances—for example when carrying outimprovements to missiles—a combination of a considerably shortercooling-down time of the IR detector with a longer cooler running timeand greater cooling loads to be transported away from the IR detectormust be achieved from these small bottle volumes at the same time.

BRIEF SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a method ofcooling a detector, in particular an IR detector in the seeker head of aguided missile as described above, which overcomes the above-mentioneddisadvantages of the heretofore-known devices and methods of thisgeneral type and which allows for a cooler running time that is as longas possible to be achieved with a restricted supply volume of workinggas.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a method of cooling a detector, themethod which comprises:

providing a pressurized fluid, the pressurized fluid being a mixture ofargon or nitrogen as a main component and at least one alkane as asecondary component, the mixture forming a positive azeotrope; and

expanding the pressurized fluid and using an expanding gas to cool thedetector.

In other words, for a method for cooling a detector, in particular an IRdetector in the seeker head of a guided missile, in which the expandinggas is used as a cooling gas and a cooling liquid to dissipate heat fromthe detector after a pressurized fluid has been expanded, in particularabove the critical point, this object is achieved according to theinvention in that a gas mixture which forms a positive azeotrope, inwhich the boiling point of the mixture is below that of the purecomponents, including argon or nitrogen as a main component and at leastone alkane as a secondary component, is expanded as the fluid.

The invention is in this case based on the consideration that thecooling gases that have been used until now with boiling points below 90K, such as nitrogen, argon, oxygen or air, do not have sufficientcooling capacity for the desired running time extension. In consequence,even when using a high-pressure bottle in which the working gas has apressure of more than 300 bar applied to it, it is not possible toachieve the desired running time extension by a factor of at least 2 to3. Their cooling capability—that is to say the integral Joule-Thomsoncoefficient—is simply too low. However, for further possible workinggases such as neon, helium or hydrogen with boiling points below 100 K,the inversion temperatures are below room temperature, of about 25° C.Expansion of working gases such as these in the given cooler conditionsand without a previous initial cooling of the high-pressure gas belowits respective inversion temperature therefore does not lead to thedesired cooling down, but to an increase in the gas temperature.Furthermore, even when using initial cooling for these gases, thecooling capability or the integral Joule-Thomson effect is too low.

Furthermore, the invention is based on the consideration that certaingases, inter alia those with a relatively high molar mass, such as thealkanes, admittedly have considerably greater cooling capacities interms of the Joule-Thomson effect, but their boiling points are allabove 100 K. It is therefore impossible to use pure alkanes as a coolinggas for cooling the IR detector to temperatures of at least 100 K.

Finally, the invention recognizes the fact that the characteristic of ahigh cooling capacity with regard to the Joule-Thomson effect and adesired low boiling point taking into account the gas-mixturethermodynamics can be achieved only in a gas mixture having a pluralityof components. Specific real gas mixtures which form a so-calledpositive azeotrope, that is to say they have mixture boiling pointsbelow the boiling temperatures of the individual pure components in theT,x diagram, exist only in a few specific cases in particular because ofspecific interactions between the individual components. In this case,inter alia, a vapor pressure curve plotted over the concentration ratioof the components on a p,x diagram with a specific composition also hasa local maximum. In this context, x represents the concentration, T thetemperature and p the pressure. Furthermore, in the case of binarymixtures composed of two components, the boiling curve andvapor-pressure curve touch one another in the phase diagram. In thiscase, the gas mixture behaves like a pure gas. A mixture of thiscomposition is referred to as an azeotrope or as an azeotropic mixture:on boiling or condensation, azeotropic mixtures behave like a puresubstance. When mixtures are composed to form positive azeotropicmixtures, in which the vapor-pressure curves have a maximum, all thesepositive-azeotropic mixtures, with these very specific compositions,have boiling points which are well below those of the individual puregas components. For example, if the condensed phase of the expandedcooling gas vaporizes in a vapor area or expansion area of a cooler byheat absorption from the detector side, then this azeotropic mixtureboils like a pure gas at this reduced mixture boiling point, whichremains constant, without any change to the mixture composition. In thecase of mixtures composed of more than two gas components, depending onthe number of components, an azeotropic point with a specificconcentration no longer occurs, in accordance with the Gibb's phaserule, but a two-dimensional or multiple-dimensional “field/region” withspecific concentration areas which can be determined precisely for theindividual gas components. Since azeotropic mixtures arepressure-dependent, the respective azeotropic mixture must be determinedfor the pressure ranges that occur in the vapor area, around 1 to 3 bar(0.1-0.3 MPa).

Extensive experiments have now shown that a mixture comprising argon ornitrogen (possibly also air) as a main component and at least one alkaneas a secondary component, with the alkane or alkanes being chosen suchthat the mixture forms a positive azeotrope (with a boiling point ofbelow 100 K), behaves like a pure gas with one of the main components incomparison to the higher boiling-point components of the alkanes in theregion with the low boiling points, in only specific concentrationratios which comprise the tightly limited azeotropic composition, forcooling of an open Joule-Thomson cooler. Under the conditions which thenoccur, a dynamic equilibrium comprising a liquid phase and a gas phaseof the azeotropic mixture at the temperature that corresponds to theboiling point of this liquid/gas phase is created in the expansion area(vapor area) downstream from the expansion nozzle. Furthermore, the gasmixture comprising main and secondary components is chosen such thatthis results not only in a positive-azeotropic gas mixture but that therespective mixture boiling point is as far as possible below 100 K.

A mixture composed of gas and liquid for cooling the detector is formedafter the expansion process in the vapor area (wet vapor region). Theessential feature in this case is that a relatively large amount ofliquid and a relatively small amount of gas are produced over widepressure ranges (from 3 to 50 MPa) since the amount of liquid determinesthe cooling capability of the cooler arrangement mainly by means of thevaporization enthalpy of this azeotropic mixture. During the cooling ofthe detector, the liquid phase which is produced in the vapor area ischanged continuously to the gas phase at a constant boiling point. Thecooler performance is in this case governed mainly by the mixturevaporization enthalpy, and less by the pure convective gas cooling. Onthe high-pressure side, new gas/liquid mixture flows continuously viathe expansion nozzle into the vapor area, thus maintaining the coolingprocess. As long as a liquid phase is present, the cooling systemremains at the constantly low azeotropic mixture boiling point, in thiscase at temperatures below the required 100 K. Since the azeotropicmixture in the vapor area and composed of gas and liquid in both phaseshas the same concentration and behaves like a pure gas, the azeotropicmixture composition in the two phases does not change over time either,and the mixture boiling point therefore also remains below 100 K, eventhough the individual alkanes have boiling points of well over 100 K.The expanded gas, like the gas which is created in the vaporizing amountof liquid as well, flows through the reverse-flow heat exchanger out ofthe vapor area into the surrounding area, in order to initially cool thehigh-pressure inlet. The gas phase from the vapor area is finallydissipated to the exterior as consumed gas mixture. At the same time,these azeotropic mixtures with the main components of nitrogen or argonmake use of the greater cooling capacity of an alkane as a secondarycomponent in comparison to nitrogen or argon, by virtue of the higherJoule-Thomson coefficients, that is to say cooling capacities.Furthermore, the higher molar mass of most of the alkanes that are usedhere is also used such that an even longer running time of theJoule-Thomson cooler can be achieved overall, with the same volumetricamount of pressurized fluid.

In this case, the expression of fluid in the pressure vessels means theaggregate state of the pressurized gas mixture above the critical pointon a T,s diagram, at which no separation of liquid from gas is evident,that is to say there is no meniscus. In this case, T represents thetemperature and s the entropy. The individual gas components nitrogen,argon and the various alkanes have a maximum in their cooling capacityat one specific pressure, that is to say Joule-Thomson coefficient,which is generally in the range from 200 to 400 bar (20-40 MPa) at roomtemperatures. However, the individual gas components exist in themixture only below their partial pressure, which is below the totalpressure, corresponding to the gas-mixture composition. The gas mixturesin the high-pressure bottle must thus be at a higher total pressure inorder to optimize the cooling process in order to ensure that theindividual gas components, at their partial pressure, approach theregion of optimum cooling performance as much as possible. This maynecessitate pressures of up to 500 bar and possibly even of 800 bar inthe pressure vessels. Higher pressures also at the same time meangreater available amounts of gas in the gas container and thereforeadditionally a longer cooler running time, as well.

The use of alkanes also offers the advantage that, as a result of thehigher Joule-Thomson coefficients, the cooling process collapses only atconsiderably low residual pressures in a high-pressure supply, thusresulting in longer cooler running times in comparison to nitrogen andargon, since the gases that remain in the high-pressure bottle can beused down to lower pressures for cooling purposes.

The use of alkanes furthermore offers the advantage that a large numberof organic impurities which originate, for example, from gas production,from compression or from a pressure bottle or from pipeline systems, aredissolved in the gas and therefore cannot be precipitated adjacent tothe expansion restrictor, and therefore cannot end the cooling processby blocking the restrictor.

In one advantageous refinement of the invention, the boiling point ofthe azeotrope should be below 100 K, in particular below 90 K. Dependingon the chosen mixture, it is sufficient in this case for the vaporpressure curve of the selected gas mixture on the p,x diagram to have amixture-specific local maximum. The boiling point of the mixture on theT,x diagram then has a minimum and is below that of the pure gases thatare involved. In the case of mixtures comprising a plurality of gases,the mixture boiling point—for the mixtures chosen here—should not besignificantly higher than that of the main component nitrogen or argon,that is to say between 85 and 100 K.

In order to prevent a component in the mixture from freezing duringexpansion of the gas mixture, which results in a temperature reduction,and which to this extent can lead to undesirable accumulation on theexpansion nozzle, it is advantageous for the sought mixture to be chosenfrom main and secondary components such that the required azeotropic gasmixture also has a composition in the vicinity of the eutecticcomposition. For this purpose, it is absolutely essential for thevarious gas components to be soluble in one another in the liquid,condensed phase. In order to ensure that the solid phase is not left,which could therefore lead to blocking of the gas flow, during theexpansion adjacent to the nozzle, the individual gas components must besoluble in one another in the condensed liquid state; this means:component a is soluble in component b in a dual mixture, component c issoluble in at least one of the components a and b in a mixture of threeitems, component d is soluble in at least one of the components a, b andc, etc. It is therefore necessary not only to determine a specificmixture composition for a positive azeotrope but, at the same time, themixture composition must also be designed so as to create a eutectic inorder that the boiling point of the fluid that is completely in solutionassumes a lower freezing point temperature than the associated boilingpoint of the mixture.

A mixture with a eutectic composition of its components is,specifically, characterized in that the melting point of the mixture islower than the melting points of the pure components. This is also animportant precondition for use of azeotropic mixtures in Joule-Thomsoncoolers, since the freezing and melting points of the individual alkanesused here are higher than the boiling point of the azeotropic gasmixture with these alkanes. In order to prevent accumulation on theexpansion nozzle as a result of individual gas components freezing inthis case, the proposed azeotropic gas mixture must preferably also havea eutectic composition: in the case of the gas mixtures proposed here,the mixture melting point must preferably remain below the mixtureboiling point. No individual solid aggregate state may occur in themixture liquid phase, that is to say no individual components aredeposited adjacent to the expansion nozzle. Mixed phases, in which oneof the components is in the liquid phase and the other component is inthe solid aggregate state, do not exist for a mixture with a eutecticcomposition. If the azeotrope thus has a composition in the vicinity ofthe eutectic composition, then this prevents individual components fromfreezing, for example the secondary components in the mixture,significantly reducing the proportion of the component which freezesout.

In particular, a melting point of below 90 K, or even better below 85 Kis advantageous for a eutectic composition of the sought mixture.

At the moment, IR detectors are cooled using gas mixtures which incombination with one another achieve two or all three of the followingeffects at very high gas pressures and with limited availablehigh-pressure gas containers, by virtue of the higher Joule-Thomsoncoefficients associated with them:

i. allow longer running times from the limited volume, or

ii. allow shorter detector cooling-down times, or

iii. allow a greater cooling load.

The fluid preferably has an initial pressure of more than 100 bar, inparticular of more than 300 bar, in particular of more than 500 bar, andin particular preferably more than 800 bar from a compressed-gascontainer of limited availability applied to it, in order that thepartial pressures of the individual gas components are in the optimumpressure range for their respective cooling capacity. The maximuminitial pressure to be provided is therefore governed by the individualgases, their molar components in the mixture and thus their variouspartial pressures. The total pressure should be chosen as a function ofthe gas composition such that the specific partial pressures of theindividual gases come as close as possible to the maximum specificJoule-Thomson coefficient. This necessarily leads to quite high initialpressures for the gas mixture. The amount of stored fluid is increasedby appropriately high compression, with a positive effect on the runningtime of the Joule-Thomson cooler. A pressure of up to more than 500 bar(in some cases even of up to 800 bar) can be applied in a compact formto the fluid by use of a high-pressure gas bottle. Pressure bottles witha maximum filling pressure of 350 bar are available without problems asstandard equipment, and pressure bottles of up 800 bar are evenavailable for trials purposes.

In one advantageous refinement, a mixture comprising 30 to 70% by volumeof nitrogen and 20 to 80% by volume of methane is used as a fluid. Eventhis simple mixture results in operating temperatures of an IR detectorto be cooled of below 100 K and extends its running time by a factor of2, in comparison to the use of pure nitrogen. Ethane is preferably addedas a further secondary component to this fluid, making up a proportionof 10 to 40% by volume. The other components, that is to say nitrogenand methane, in this case make up proportions of 20 to 40% by volume and10 to 40% by volume, respectively. Once again, the boiling point remainsbelow 100 K here, although the running time extension is in this casemore than a factor of three.

A mixture comprising 30 to 70% by volume of nitrogen, 15 to 35% byvolume of ethane and 15 to 35% by volume of propane has been found to beanother suitable mixture for use as the fluid. In order to furtherreduce the achievable low temperature and greater cooling capacity, aproportion of 10 to 30% by volume of methane can be added to thismixture, as a further component. The other components, that is to saynitrogen, ethane and propane, in this case make up proportions of 20 to70% by volume, 10 to 25% by volume and 10 to 20% by volume,respectively. In all situations which lead to a positive-azeotropic andeutectic mixture, the boiling point of the gas mixture is below 100 K,no solid phase occurs, and the running time is extended by a factor of 3to 4 in comparison to that of nitrogen, depending on the ambienttemperature.

All the gas mixtures mentioned here for running time extension inJoule-Thomson coolers additionally provide significantly shortercooling-down times, greater cooling loads and combinations thereof, as aresult of the greater cooling capacity, and this may be a criticalfactor for certain missile types.

In one alternative refinement, a mixture comprising 45 to 60% by volumeof argon and 35 to 50% by volume of methane is used as the fluid. Theazeotrope in this mixture, which has a composition of 56% by volume ofargon and 44% by volume of methane, admittedly has a slightly higherboiling point of about 96 K than argon (argon has a boiling point of87.3 K), but the boiling point has been reduced sufficiently incomparison to methane such that a wet vapor mixture of the azeotropiccomposition occurs in the expansion area, with a boiling point of below100 K. This results in a fluid which can be used, and which makes itpossible to reduce the temperature of a detector to the desiredoperating point of below 100 K.

Further details relating to fluids based on nitrogen, with alkanes beingused as secondary components, can be found in particular in GB 1 3336892. The compositions described there are, however, specified for use ina cooling circuit with low compression pressures, in particular forclosed circuits with a maximum pressure of 30 bar. It is not possible topredict their characteristics when used in non-equilibrium conditions inan open Joule-Thomson cooler.

The fluids used do not exhibit ideal behavior. Particularly at lowtemperatures, such as those which can occur during use of a guidedmissile, significant pressure reductions occur in a pressure bottle.Particularly at low temperatures of down to neg. 45° C. which missilescan be reduced to, this results in noticeable cooling performance lossessince the pressure difference for the gas which is expanded in this wayis reduced. In one advantageous refinement of the invention, thepressurized fluid is therefore temperature-stabilized. This is achieved,for example, by means of heating mats or integrated heating elements,Peltier elements or by means of existing dissipative heat sources, suchas electronics, in which case, by way of example, heat tubes can be usedfor heat transport. Temperature-stabilizing elements such as these areused in particular when the cooling power losses that occur as a resultof the pressure loss cannot be compensated for in situ by the ambienttemperature, which is low in any case. If temperature stabilizationtakes place, then the pressure loss in the real mixture can becompensated for, thus leading to a further increase in the running timeof the Joule-Thomson cooler. In general, the Joule-Thomson effect withthe gases that are used increases as the temperatures become lower. Thepressure drop in the case of a relatively cool environment can becompensated for once again by this effect (partially). However,additional temperature stabilization of the pressure vessel with arelatively cool Joule-Thomson cooler will then additionally contributeto a further running-time extension.

When the fluid from a pressurized pressure bottle is expanded, then theheating means to be used for temperature stabilization must necessarilybe arranged such that they act on the pressure bottle. By way ofexample, heating mats or the like can surround the pressure bottle.

When heating elements are used for temperature stabilization, they canlead to a pressure increase as the pressure in the pressure bottledecreases, and they can therefore be used to increase the coolingperformance of the Joule-Thomson cooler. This is achieved in particularby the heating elements heating the pressurized fluid above the ambienttemperature. A value of about 50° C. has been found to be particularlysuitable for this purpose in practice.

None of the fluid compositions mentioned is toxic; however, in certainmixture ratios, they are explosive when introduced into air containingoxygen. In order to reduce this explosive behavior, it is advantageousto admix heptafluoropropane with a content of between 5 and 15% byvolume to the fluid as a further component. In addition, alternativelyor in combination, tetrafluoromethane can be admixed to make up acontent of between 3 and 20% by volume. Both components are licensed asflame-resistant agents and can be used to replace bromiumtrifluoromethane, which is no longer permissible for environmentalreasons (in accordance with the Montreal Agreement). These twocomponents are therefore used to constrain/suppress burning of thecombustible alkanes. Since, in particular, tetrafluoromethane has aquite high cooling capability, this component can also be used to extendthe running time of the Joule-Thomson cooler and to decrease anypossible explosion risk of the alkane mixture that is used.

In one preferred refinement, the expanded gas flows in the oppositedirection to that before it was expanded, in order to cool thepressurized fluid. This refinement results in the initially mentionedreverse-flow cooling, with the fluid which is supplied to thehigh-pressure side of the expansion nozzle or restrictor being cooleddown by the returning gas flowing in the opposite direction through anappropriate reverse-flow heat exchanger, before finally emerging intothe environment.

In one advantageous development of the method, a further pressurizedfluid is expanded, with the expanding gas of the further fluid beingused to cool the pressurized fluid before it is expanded. This measureresults in a multistage Joule-Thomson cooler with the fluid that is usedto cool the detector exchanging heat with the expanding gas and thefurther fluid cooled down in this way, before emerging from theexpansion nozzle. This initial cooling means that the fluid which isused to cool the detector can be cooled down to a very low temperatureeven before it is expanded, as a result of which the further reductionin its temperature that results from this expansion process incomparison to the inversion temperature results in an improvement in thecooling performance.

In particular, the fluid which is used for cooling can be precooled bythe first expansion stage to such an extent that, at the low temperaturewhich then occurs adjacent to the expansion nozzle or restrictor, thesubsequent isoenthalpy expansion results in a specific cooling powerwhich is sufficiently great that the detector is cooled down from roomtemperature to the required detector temperature of below 100 K within ashort time. This two-stage embodiment of the cooler makes it possible inparticular to achieve cooling times for cooling down from 295 K to below100 K of less than two seconds. The latter is now a requirement forguided missiles whose IR detectors must be cooled down to the operatingtemperature within this time in order, for example, to make it possibleto detect targets flying at supersonic speed, sufficiently quickly.

Since the fluid which is used for cooling is precooled even before it isexpanded, there is no longer any need to carry out additional precoolingby means of a reverse-flow heat exchanger using the returning expandedgas. In particular, the fluid which has been expanded and has beencooled down to its boiling point can be sprayed onto the rear face ofthe detector to be cooled, during the expansion process, in the form ofa spray coolant. The latter allows a cooler design without anymechanical link to a moving detector.

However, the temperature of the fluid can be reduced further before itsexpansion by the expanded gas of the further fluid flowing in theopposite direction to that before it was expanded, in order to cool thepressurized fluid. In this case, the fluid which is used for cooling notonly makes thermal contact with the vapor area of the first expansionstage before its expansion, but its inlet is additionally cooled by theexpanded gas of the further fluid flowing back.

In a further preferred refinement, the expanded gas of the further fluidflows in the opposite direction to that before it was expanded, also inorder to cool the pressurized further fluid.

This makes it possible to further reduce the temperature which can beachieved in the vapor area of the first expansion stage.

Because of the high integral Joule-Thomson coefficients, methane and inparticular tetrafluoromethane (also referred to as tetrafluorocarbon)can be used as the further fluid for the first cooler stage. However,argon can also be used for the same reason. Argon has a cooling capacitythat is greater than that of nitrogen by a factor of 1.5.

In an alternative refinement, the mixture as described above and whichforms a positive azeotrope, comprises argon or nitrogen as a maincomponent and at least one alkane as a secondary component, is also usedfor the further fluid, because of the low temperatures that can beachieved and the high cooling capacity. In this case, the describedrefinements and compositions can likewise preferably be used for themixture.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin method for cooling a detector, it is nevertheless not intended to belimited to the details shown, since various modifications and structuralchanges may be made therein without departing from the spirit of theinvention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a schematic view of a Joule-Thomson cooler;

FIG. 2 is a cross section taken through the technical implementation ofa Joule-Thomson cooler;

FIG. 3 is a graph illustrating the enthalpy profile during the expansionprocess in an open Joule-Thomson cooler; and

FIG. 4 is a schematic view of a two-stage Joule-Thomson cooler.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1 thereof, the schematic shows the design of anopen Joule-Thomson cooler 1 for cooling an IR detector 2. A pressurizedfluid flows from a pressure bottle or pressure tank 4 via an inlet valve6 to an inlet path 7 to a counter-flow or reverse-flow cooler 10. Thetemperature of the fluid is thereby decreased in comparison to thetemperature in the pressure bottle 4, by means of the cooler return 14.The pressurized fluid is expanded via a restrictor 11 which, inparticular, is in the form of a nozzle. The expanding gas enters anexpansion area or vapor area 13 where it is cooled down as a consequenceof the expansion process. As a result of the temperature reduction, adynamic equilibrium is created between a gas phase 16 and a liquid phase17 in the expansion area 13 at the boiling point of the azeotropiccomposition, cooling down the IR detector which is arranged at thebottom of the expansion area 13, by means of a thermal contact. In thiscase, a temperature close to the boiling point is achieved as the lowtemperature.

Gas of the composition of the gas phase flows out of the expansion areavia a return path 14 through the reverse-flow cooler 10, cooling thefluid as it flows in. After passing through the return path 14, theexpanded gas is exhausted to the environment through an outlet 18.

FIG. 2 shows a cross section of a technical implementation of an open,flow-controlled Joule-Thomson cooler 1′. In this case, the IR detector 2to be cooled adjoins the inner wall of a Dewar vessel 19. The interiorof the Dewar vessel 19 is evacuated, thus providing good thermalinsulation with respect to thermal conduction and radiation to theenvironment. A connecting stub 20 extends into the internal area of theDewar vessel 19 and is provided with a flange 22 for attachment. A gassupply line 23 is arranged in the connecting stub 20 and is connected toa pressure bottle in order to supply with a pressurized fluid. Thepressurized fluid flows along the lines which helically surround theconnecting stub 20 and form the inlet path 7, to the expansion nozzle 11where the fluid is expanded. The emerging gas expands into the expansionarea 13.

Gas in the gas phase flows out of the expansion area 13 via the lineswhich form the inlet flow path, thus forming the return path 14, and arepassed to the exterior of the upper end of the Dewar vessel 19. Theinlet flow is therefore cooled by the flow in the opposite direction.

The method of operation of an open Joule-Thomson cooler as shown inFIGS. 1 and 2 will be explained by means of the temperature-entropygraph (for argon as an example) illustrated in FIG. 3. The graph showsthe states which occur during the expansion process in the Joule-Thomsoncooler, annotated with the letters “A” to “D”. The associated points aremarked in a corresponding manner in the schematic illustration of theJoule-Thomson cooler in FIG. 1.

The entropy of the system is plotted on the abscissa of the graph. Thesystem temperature or system lines of equal enthalpy are marked on theordinate. The graph also shows isobars with a pressure of p=1000 bar,p=500 bar, p=300 bar and p=1 bar. The curve profiles of constantenthalpy are also shown on the graph.

Starting with a fluid which has been pressurized to a pressure of p=500bar and is at a temperature of 350 K at the point B, the fluid flows, asshown in FIG. 1, through the inlet path 7, where it is pre-cooled by theexpanded and cooled-down gas flowing back in the opposite direction. Thepressure along the inlet path 7 to the expansion nozzle 11 can in thiscase be considered to be constant. In consequence, the system as shownin FIG. 3 starts from the point B and moves on a curve of constantpressure of p=500 bar to a point C of low temperature.

The fluid is expanded at the expansion nozzle 11. The emerging gasexpands as shown in FIG. 1 into the expansion area 13. During thisexpansion process, the gas is cooled down along a curve of constantenthalpy. The system state in this case moves as shown in FIG. 3 frompoint C to point D in the wet vapor region, with the gas emergingpartially in the liquid aggregate state. Based on the lever law, anamount of liquid in the ratio D-D″ and a corresponding amount of gas inthe ratio D-D′ are produced. In the expansion area, the liquid phaseexists in accordance with the state point D′ in an equilibrium with thegas phase D″. The detector 2, which makes thermal contact with theexpansion area 13, is cooled down to a temperature of below 100 Klargely by the amount of liquid.

The gas flows from the gas phase D″ at a normal pressure of about p=1bar outwards by the return path 14. During the process, the gas flowingout in the return path 14 is heated by heat dissipation from the fluidflowing in the inlet path 7. As shown in FIG. 3, the system moves in acorresponding manner on a curve of constant pressure from P=1 bar to thepoint A at the ambient temperature of 350 K.

If the curve of constant enthalpy is considered, starting from the pointB, then this results in the point E. After the gas emerges from theJoule-Thomson cooler, the overall system has an increased enthalpy ofthe point A. The reversible cooling power of the Joule-Thomson cooler iscalculated from the enthalpy difference at the points A and E. Thisenthalpy difference is in the ideal case taken from the detector ascooling power and from the environment as dissipated energy.

A number of experiments were carried out using a Joule-Thomson cooler asshown in FIG. 2, with different fluid mixtures in a temperature rangebetween −54° C. and +70° C. In this case, a pressure tank was used witha volume of 415 ccm at an initial pressure of 345 bar, and at atemperature of 220. A fluid I with 30% by volume of nitrogen, 30% byvolume of methane, 20% by volume of ethane and 20% by volume of propane,as well as a fluid II with a proportion of 30% by volume of nitrogen,35% by volume of methane and 35% by volume of ethane were investigatedas fluid mixtures. In contrast to argon and air as pure cooling gases,the behavior of the fluid mixtures was now investigated in terms of therunning time of the Joule-Thomson cooler. The running time was in thiscase investigated with a pressure bottle at temperatures of −54° C.,+22° C. and +70° C. A glass Dewar was used as the Dewar vessel 19 asshown in FIG. 2, in order to analyze the processes in the expansion area13.

The same experiments were carried out by a fluid III with a compositionof 56% by volume of argon and 44% by volume of methane, as well as afluid IV composed of a mixture of 70% by volume of nitrogen and 30% byvolume of methane.

The result of these experiments is that it can be stated that a runningtime extension is found in all the investigated temperature ranges withthe fluids I, II, III and IV that were used, with an achieved coolingtemperature below 100 K, in comparison to air and argon. In this case,the fluid I exhibited the greatest running time extension. The extensionfactor was in this case 2.6; 4.4 and 4.4, respectively, in comparison toair and 1.9; 2.7 and 2.9, respectively, in comparison to argon at thetemperatures −45° C., +22° C. and +70° C. At the investigatedtemperature of 22° C., in comparison to argon, the fluid II resulted ina running time extension by a factor of 2.4, with a factor of 4.0 incomparison to air.

Overall, it was possible to achieve running times of between 4 and 8hours using the reference cooler comprising a pressure vessel of only415 cm³ and with an initial pressure of 340 bar at room temperatures,and of between 4 and 11 hours at temperatures of +70° C. The runningtimes could be increased even further by higher initial pressures andtemperature-stabilized pressure vessels.

FIG. 4 shows, schematically, the design of a two-stage Joule-Thomsoncooler 38 with a fluid which cools an IR detector 80 by means ofexpansion and comprises a mixture forming a positive azeotrope beinginitially cooled by expansion cooling of a further fluid.

The Joule-Thomson cooler 38 illustrated in FIG. 4 is split, in order toassist understanding, into two coolers 40 and 42, but these should notbe confused with the expansion stages.

The first cooler 40 is in this case operated with a mixture, forming apositive azeotrope, from a compressed-gas container 44. The mixture usedin the compressed-gas container 44 is at ambient temperature and at apressure of 200-500 bar. The mixture is passed by a valve 46 and astraight line 48 running through the cooler 42 to an inlet path 50 of aheat exchanger 51 of the cooler 40. The first cooler 40 is an expansioncooler with an expansion nozzle or restrictor 52. The restrictor 52 isconnected to the output of the inlet path 50 via a high-pressure line54. The high-pressure line 54 is provided with thermal insulation 56.

The second cooler 42 is operated with tetrafluoromethane from acompressed-gas container 58. The tetrafluoromethane in thecompressed-gas container 58 is likewise of ambient temperature and at apressure of 200-350 bar. The tetrafluoromethane is passed via a valve tothe input 62 of an inlet path 64 of a reverse-flow heat exchanger 66 inthe second cooler 42. A line 70 passes from the output 68 of the inletpath 64 of the reverse-flow heat exchanger 66 straight through thesecond cooler 40 to a restrictor or expansion nozzle 72. The restrictor72 is seated at the end of the first cooler 40 that is remote from thesecond cooler 42. The tetrafluoromethane, which is at high pressure,emerges from the restrictor 72. In the process, it is expanded and iscooled down. The expanded and cooled-down tetrafluoromethane now flowsthrough a return path 74 through the heat exchanger 51 in the firstcooler 40 in the opposite direction to the mixture which is flowing inand forms a positive azeotrope. This mixture is therefore precooled inthe first cooler 40 by the expanded tetrafluoromethane wet vapor, butnot by the expanded mixture itself. The expanded tetrafluoromethane thenflows through a return path 76 through the reverse-flow heat exchanger66 in the second cooler 42. Here, the tetrafluoromethane which isflowing in and is at high pressure is precooled by the expanded andcooled-down tetrafluoromethane. The expanded tetrafluoromethane emergesfrom the return path 76, at an outlet 78.

The mixture which flows out, is used for cooling and forms a positiveazeotrope, is aimed in a jet at an IR detector 80 which is arranged in amoving mount 82. The expanding gas from this mixture then emerges fromthe mount 82 through an aperture 84.

The two coolers 40 and 42 are surrounded by a casing 86 which is closedon the object side by an end wall 88. The thermally insulatedhigh-pressure line 54 is passed through the end wall 88.

The fluid III as described above and as investigated, and comprising 56%by volume of argon and 44% by volume of methane is particularly suitablefor use as a mixture for cooling down the IR detector 80. This mixturehas a boiling point of about 96 K (at 1 bar) and a melting point of lessthan 75 K. The cooling power is better than that of argon by a factor ofabout 2. The second expansion stage (associated with the first cooler40) can also be operated with a mixture comprising 30-70% by volume ofnitrogen, 15-35% by volume of propane and 15-35% by volume of ethane. Amixture comprising 40% by volume of nitrogen, 30% by volume of propaneand 30% by volume of ethane results, in comparison to nitrogen, in acooling capacity that is about 3 to 7 times greater with a boiling pointof only 78 K (at 1 bar). No freezing of the expansion nozzle was found.In comparison to the argon which was also used, the mixed gas resultedin a somewhat higher boiling point, with a cooling capacity that wasbetter by a factor of 2 to 4.5 times.

Furthermore, it is also possible to use a mixture comprising 50-64% byvolume of nitrogen and 36-50% by volume of methane. A mixture with thesame proportions of nitrogen and methane has a boiling point of 82 K (at1 bar). The mixture remains liquid at the boiling point of nitrogen. Asour own measurements have shown, the mixture results in a coolingcapacity which is about twice as good as that of pure nitrogen.

Furthermore, it is also possible to use a mixture comprising 20-70% byvolume of nitrogen, 20-40% by volume of methane and 10-40% by volume ofethane. Since methane is soluble in liquid nitrogen, ethane is solublein liquid methane, and ethane and propane are soluble in one another,this mixture has an even better cooling capacity. In particular, amixture comprising 30% molar of nitrogen and 35% molar of methane andethane, respectively, has a cooling capacity which is 4 to 9 timesgreater than that of nitrogen. The boiling point of this mixture isabout 80 K. This mixture behaves like an azeotropic mixture, and has thecharacteristics of a virtually eutectic mixture, since no freezingoccurs at the low boiling point.

Furthermore, a mixture comprises 20-70% by volume of nitrogen, 10-30% byvolume of methane and 10-25% by volume of ethane and propane,respectively, also has good characteristics. The boiling point of amixture comprising 30% by volume of nitrogen, 30% by volume of methaneand 20% by volume of ethane and propane, respectively, is about 80 K (at1 bar). The cooling capacity is better than that of nitrogen by a factorof 7 to 12.

1. A method of cooling a detector, the method which comprises: providinga pressurized fluid, the pressurized fluid being a mixture of argon ornitrogen as a main component and at least one alkane as a secondarycomponent, the mixture forming a positive azeotrope; and expanding thepressurized fluid and using an expanding gas to cool the detector. 2.The method according to claim 1, which comprises cooling an IR detectorin a seeker head of a guided missile.
 3. The method according to claim1, wherein a boiling point of the azeotrope is below 100 K.
 4. Themethod according to claim 3, wherein the boiling point of the azeotropelies below 90 K.
 5. The method according to claim 1, which comprisesexpanding a mixture with an azeotrope having a composition in a vicinityof a eutectic composition thereof.
 6. The method according to claim 1,which comprises choosing the components of the mixture to be completelysoluble in one another in a condensed liquid phase, with at least oneliquid component being soluble in another liquid component.
 7. Themethod according to claim 5, wherein the eutectic composition of themixture has a melting point below 90 K.
 8. The method according to claim1, which comprises providing the pressurized fluid at a pressure of morethan 100 bar.
 9. The method according to claim 1, which comprisesproviding the pressurized fluid at a pressure of more than 300 bar. 10.The method according to claim 1, which comprises providing thepressurized fluid at a pressure of above 800 bar.
 11. The methodaccording to claim 8, which comprises choosing an initial pressure ofthe mixture such that partial pressures of individual gases are in apressure range of a respective optimum integral Joule-Thomsoncoefficient of each individual gas corresponding to a molar compositionin the mixture.
 12. The method according to claim 8, which comprisesproviding a mixture of 30-70% by volume of nitrogen and 20-80% by volumeof methane.
 13. The method according to claim 1, which comprisesproviding a mixture comprising 20-70% by volume of nitrogen, 20-40% byvolume of methane, and 10-40% by volume of ethane.
 14. The methodaccording to claim 13, wherein the mixture comprises 30-70% by volume ofnitrogen, 15-35% by volume of ethane, and 15-35% by volume of propane.15. The method according to claim 1, wherein the mixture comprises20-70% by volume of nitrogen, 10-30% by volume of methane, 10-25% byvolume of ethane, and 10-25% by volume of propane.
 16. The methodaccording to claim 1, wherein the mixture comprises 45-60% by volume ofargon and 35-50% by volume of methane.
 17. The method according to claim1, which comprises temperature-stabilizing the pressurized fluid. 18.The method according to claim 17, which comprises stabilizing the fluidat a temperature above room temperature.
 19. The method according toclaim 17, which comprises expanding the fluid from a pressurized bottle,and stabilizing the temperature of the fluid by way of a heating elementacting on the pressurized bottle.
 20. The method according to claim 1,which comprises admixing between 5 and 15% by volume ofheptafluoropropane to the fluid as a further component.
 21. The methodaccording to claim 1, which comprises admixing between 3 and 20% byvolume of tetrafluoromethane to the fluid as a further component. 22.The method according to claim 1, which comprises conducting the expandedgas to flow in counterflow to the pressurized fluid prior to expansion,to thereby cool the pressurized fluid.
 23. The method according to claim1, which comprises expanding a further pressurized fluid to form afurther expanding gas, and cooling the pressurized fluid with thefurther expanding gas prior to expanding the pressurized fluid.
 24. Themethod according to claim 23, which comprises conducting the furtherexpanded gas of the further pressurized fluid flows in counterflow tothe pressurized fluid for cooling the pressurized fluid.
 25. The methodaccording to claim 23, which comprises conducting the further expandedgas in counterflow to the further pressurized fluid, to cool the furtherpressurized fluid prior to expanding the further pressurized fluid. 26.The method according to claim 23, which comprises spraying the expandinggas of the fluid against the detector.
 27. The method according to claim23, wherein the further pressurized fluid is tetrafluoromethane.
 28. Themethod according to claim 23, which comprises using a common fluid forthe pressurized fluid and the further pressurized fluid.